One example of conventional power supply apparatuses has three-phase armature windings provided to an armature core of a stator, and a field winding provided to a magnetic pole core of a rotor, and includes: a three-phase AC generator that is driven by rotation of an engine of a vehicle or the like and thereby generates electricity; a rectifier that converts three-phase AC voltages generated by the three-phase AC generator into DC voltages through rectification; and a field adjusting circuit that detects output voltages of the rectifier and adjusts an exciting current flowing in the field winding such that the output voltages will be a predetermined value (see, for example, Patent Literature 1).